High voltage current limiting fuse

ABSTRACT

A current limiting fuse structure comprising a generally tubular, electrically insulating casing having terminal means disposed adjacent to the opposite ends thereof. One or more fusible elements or links is connected between the terminal means of said fuse structure. An axially extending electrically insulating member is disposed on one or more of the fusible elements between at least the central portion thereof and one of the associated terminal means. The electrically insulating member is formed from a material which evolves one or more gases which assist in arc extinction in the presence of an arc which results when the associated fusible element melts or blows. The last mentioned material is also substantially non-tracking from the electrical standpoint in the presence of an arc.

United 'States Patent [191 Cameron HIGH VOLTAGE CURRENT LIMITING FUSE [75] Inventor: Frank L. Cameron, Irwin, Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Sept. 30, 1971 [21] Appl. No.: 185,199

[52 US. Cl. 337/279, 337/296 [51] Int. Cl. II0lh 85/44 [58] Field of Search 337/159, 160, 161,

[56] I References Cited I UNITED STATES PATENTS 2,143,031 1/1939 Rapp .L 337/279 FOREIGN PATENTS OR APPLICATIONS 441,600 l/l936 Great Britain 337/279 [451 Oct. 16, 1973 Primary Examiner-Bernard A. Gilheany Assistant Examiner-F. E. Bell AttorneyA. T. Stratton et al.

TSTJ ABSTRACT A current limiting fuse structure comprising a generally tubular, electrically insulating casing having termi- 'nal means disposed adjacent to the opposite ends thereof. One or more fusible elements or links is connected between the terminal means of said fuse structure. An axially extending electrically insulating member is disposed on one or more of the fusible elements between at least the central portion thereof and one of the associated terminal means. The electrically insulating member is formed from a material which evolves one or more gases which assist in arc extinction in the presence of an are which results when the associated fusible element melts or blows. The last mentioned material is also substantially non-tracking from the electrical standpoint in the presence of an arc.

10 Claims, 5 Drawing Figures 1 HIGH VOLTAGE CURRENT LIMITING FUSE CROSS REFERENCES TO RELATED APPLICATIONS The invention disclosed and claimed in the present application is related to that which is disclosed and claimed in my copending application Ser. No. 185,201 filed concurrently with this application.

BACKGROUND OF THE INVENTION In the construction of current limiting fuses, particularly those of the high voltage type, the cross-sectional size or area of each of the fusible elements employed is normally relatively small and each fusible element or link usually includes portions of reduced cross-section to provide the desired current limiting action. In certain applications of such fuses, such as at relatively high voltages which may be 5 KV and above, current limiting fuses may be required to interrupt overload currents over a full or relatively wide range of overload currents which extends from relatively low overload currents to relatively high overload currents. If each fusible element in a particular fuse structure includes a plurality of axially spaced reduced cross-sectional portions, a plurality of series arcs results which facilitate the final interruption of the overload current when the fuse structure is called upon to interrupt relatively high overload currents, but usually only a single are results when a relatively low overload current occurs.

Various means have been proposed to facilitate the interruption of relatively low overload currents in a current limiting fuse structure of the type described. One approach is to employ numerous fusible elements, each having a very small cross-sectional area. This approach may result in a relatively higher resistance of the overall fuse structure. Another means proposed to facilitate the interruption of relatively low overload currents is to provide one or more auxiliary fusible elements with electrically insulating gaps between each auxiliary fusible element and an associated main fusible element as disclosed in U.S. Pat. No. 3,243,552. A third means proposed for the above mentioned purpose is to provide a support member for the fusible elements which is at least partially gas-evolving in the presence of an arc to aid in arc extinction, as disclosed in U.S. Pat.-No. 3,569,891 and is partially shown in FIG. 1A. A fourth means proposed to solve the above mentioned problem is to provide each fusible element with associated gas-evolving plates which form expulsion pots during an interrupting operation, as disclosed in U.S. Pat. No. 3,020,372 and as shown partially in FIG. 1B. A fifth means proposed to facilitate low current interruption in a fuse of the type described is to provide a layer or zone of gas-evolving, non-fulgurite producing filler material which is particularly effective in facilitating low current interruption, as disclosed in my U.S. Pat. No. 3,213,242 and as shown partially in FIG. 1C.

SUMMARY OF THE INVENTION In accordance with the invention, a current limiting fuse structure includes a generally tubular electrically insulating casing or housing having terminal means mounted on said casing adjacent to each of the oppo site ends of the casing. One or more fusible elements is disposed in the casing and electrically connected between the associated terminal means. An elongated electrically insulating member is disposed on and secured to one or more of the fusible elements between the axially central portion thereof and at least one of the associated terminal means. The insulating member extends substantially around and axially along a portion of the associated fusible element, and is formed from a highly compressed material, such as boric acid, which is isostatically pressed on said fusible element, which is adapted to withstand relatively high temperatures and to evolve one or more gases which aid in arc extinction in the presence of an are, which results when the associated fusible element melts. A pulverulent arc quenching filler material is disposed in said casing in contact with'at least a portion of each fusible element to additionally aid in arc extinction.

Where desired, each fusible element provided may be formed from electrically conducting fusible material of the flat ribbon type having periodically spaced restricted or reduced portions along its length. In addition, in an illustrated embodiment of the invention, an M" effect causing means, more specifically, a quantity of a low melting point metallic alloy, such as tin-lead, may be disposed on each fusible element adjacent to the central portion thereof and adjacent to one end of the electrically insulating member previously mentioned which is disposed on the fusible element.

In another embodiment of the invention, an additional electrically insulating member of the same material as described above may be disposed on and secured to each fusible element between the central portion thereof and the other terminal means of the overall fuse structure, which results in two or more axially spaced electrically insulating members, as described above, on a particular fusible element.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, reference may be had to the preferred embodiment exemplary of the invention shown in the accompanying I DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings and FIGS. 1A, 1B and 1C, in particular, there are illustrated portions of three prior art current limiting fuse structures 12, 14 and 16 of the types previously mentioned. The fuse structure 12 shown in FIG. 1A includes a fusible element 13 which is mounted or supported on an associated electrically insulating support member 15 which is at least partially formed from a material which evolves one or more gases in the presence of an arc to aid in are extinction. The portion of the fuse structure 14 shown in FIG. 18 includes a fusible element 17 on which is mounted the electrically insulating plates 19 which may evolve one or more gases in the presence of an arc to aid in arc extinction. The portion of the fuse structure 16 shown in FIG. 1C includes a casing 18 in which the fusible element 11 is disposed and a filler material 23 disposed around at least the central portion of the fusible element 11 to evolve gases during the interruption of an are but not to form a fulgurite in the presence of an are, as disclosed in US. Pat. No. 3,213,242 previously mentioned. The fuse structure 16 also includes granular filler material such as sand, as indicated at'21 on axially opposite sides of the filler material 23 to additionally aid in arc extinction during certain operating conditions and to form a fulgurite when the associated fusible element 11 melts, as disclosed in the latter patent.

Referring to FIG. 2, there is illustrated a current limiting fuse structure 10, which is particularly adapted for high voltage applications, such as KV and above, and which embodies the principal features of the invention. As illustrated, the fuse structure includes a generally tubular casing or housing 20, which is formed from a suitable electrically insulating material which has sufficient structural and mechanical strength to withstand the thermal conditions and internal pressures which may'result during the operation of the fuse structure 10, such as a glass-reinforced melamine or epoxy resin material. In order to close off the opposite ends of the casing and to provide means for making electrical connections to the fuse structure 10 adjacent to the ends thereof, the terminal end caps or electrically conducting ferrules 32 and 34 are secured to the opposite ends of the casing 20 by suitable means, such as the magnetic forming method which is described in detail in US. Pat. No. 3,333,336, which issued Aug. 1, 1967 to F. L. Cameron and W. C.'Good, and which is asa signed to the same assignee as the present application.

When desired, the axially projecting electrically conducting studs 52 and 54 maybe mounted on or integrally formed with the end caps 32 and 34, respectively, in order to permit the mounting of the fuse structure 10 in particular types of supporting structures. As illustrated, the fuse structure '10 also includes the internal terminal members 62 and 64 which are generally disposed between the opposite ends of the casing 20 and their respective end caps32 and 34. Each of the terminal members 62 and 64 may be formed from a suitable electrically conducting material, such as copper or a copper alloy, and as illustrated, may be washershaped in configuration with a central opening through which one end of the associated fusible element passes.

In general, the fuse structure 10 includes one or more fusible elements or links 30, which are electrically connected and extended axially betweenthe terminal end caps 32 and 34 or between the terminal members 62 and 64, as shown inFIG. 2. As'illustrated, the fuse structure '10 includes .a single fusible element 30 which is electrically connected between the terminal members 32 and 34. More specifically, the fusible element 30 may be formed from a predetermined length of electrically conducting fusible material, such as silver of the flat ribbon type and including a plurality of axially spaced points or portions of reduced or restricted cross-sectional area, as indicated at 30A, which may be formed by V-notching the ribbon material from which the fusible element 30 is formed on both sides at axially spaced points along its length. In order toprovide the" 30 may be assembled to pass through the central openings in the associated terminal members 62 and 64 prior to the assembly of the associated end caps 32 and 34, respectively, over the ends of the casing 20, and then be clamped or secured between the end caps 32 and 34 and their respective terminal members 62 and 64.

5 As shown in FIG. 2, the fusible element 30 may have disposed on the central portion thereof a ball or head of M effect causing material or means as indicated at 36, which comprises a quantity of low melting point metallic alloy, such as a combination of tin and lead. The effect of the low melting point ball 36 is to modify the low current melting characteristics of the fusible element 30 and to improve the ability of the fuse structure 10 to interrupt relatively low overload currents at high voltages. It is to be noted that the number of fusible elements 30 required in a particular application depends upon the current rating of the fuse structure 10 with additional fusible elements similar to the fusible element 30 being electrically connected in parallel to provide the necessary continuous current rating of the overall fuse structure 10.

In order to facilitate the interruption of relatively lower overcurrents during the operation of the overall fuse structure 10, the fuse structure 10 as shown in FIG. 2 also includes an elongated, electrically insulating member 40 which is disposed on and secured to the fusible element 30. The electrically insulating member 40 is disposed to substantially surround the associated fusible element 30 and to extend axially along the fusible element 30 from a location or point which is adjacent to the central portion of the fusible element 30 toward one of the associated terminal members, which in this case is the upper terminal member 62 or the upper end cap 32, as viewed in FIG. 2. Where the M effect causing means 36 is disposed on the fusible element 30, one end of electrically insulating member 40 may be disposed adjacent to the M" effect causing means 36 as shown in FIG. 2. The electrically insulating member 40 is preferably formed from electrically insulating normally solid material, which is adapted to evolve one or more gases which aid in arc extinction or interruption in the presence of any are which results when the associated fusible element 30 melts or fuses, which is substantially non-tracking in the presence of any are which results during the operation of the fuse structure 10, and which is adapted to withstand the high temperatures or thermal conditions and pressures which may result during the operation of the fuse structure 10 without disintegrating or breaking up. It is also important to note that the material from which the electrically insulating member 40 is formed is preferably one which does not form a fulgurite during an interrupting operation of the fuse structure 10. An example of such material which has been found to be particularly effective in practicing the disclosed invention is highly compressed boric acid material, which evolves water vapor when exposed to an arc during the operationof the fuse structure 10. The desired density of the material from which the electrically insulating member 40 may be formed may be obtained by isostatically compressing the material directly on the associated fusible element 30 until theminimum hardness of the boric acid material, for example, is about 35 on the D scale of a wellknown hardness measuring instrument, which is sold under the trade name Shore Durometer. To achieve such a hardness of the boric acid material requires that the boric acid material be compressed by a minimum isostatic pressure of approximately 25,000 lbs. per square inch. When the electrically insulating member 40 is formed on the associated fusible element 30 in the manner just described, the electrically insulating member 40 is secured to the fusible element 30 and is disposed in intimate contact with an axially extending portion of the fusible element 30 to provide a controlled amount of gas evolution which particularly aids in arc extinction during certain operating conditions of the fuse structure 10, as will be described hereinafter. It is to be noted that the amount of gas evolution desired in a particular application may be controlled by varying the axial dimension of the electrically insulating member 40.

In order to additionally aid or assist in arc interruption during certain operating conditions of the fuse structure and to assist in providing the current limiting action which is necessary in a fuse structure of the type described, the space between the casing and the fusible element and between the casing 20 and the electrically insulating member is substantially filled with a finely divided, pulverulent or granular arcquenching filler material, as indicated at 72 in FIG. 1, such as silica sand or quartz sand, in which the fusible element 30 and the electrically insulating member 40 are effectively embedded It is to be noted that after the casing 20 is substantially filled with the arc-quenching material 72, the tiller material 72 is then preferably compacted by any suitable means such as vibration or other known methods. It is also to be noted that the presence of the electrically insulating member 40, which substantially surrounds a portion of the fusible element 30, prevents the formation of a fulgurite along an axially extending portion of the fusible element 30 during an interrupting operation of the fuse structure 10, which might otherwise result if the filler material 72 were in contact with the portion of the fusible element 30, which is substantially surrounded by the electrically insulating member 40.

In the operation of the overall fuse structure 10, as illustrated, when a relatively low abnormal or overload current starts to flow through the fusible element 30 of the fuse structure 10, the axially central portion of the fusible element 30 will begin to melt initially after a relatively long time overload condition which may continue for as long as approximately 30 minutes in a particular application. It is to be noted that where the ball of low melting point material 36 is disposed on a central portion of the fusibleelement 30, the ball of material 36 will cause the fusible element 30 to melt or to fuse at a temperature of about 182 to 232 C. Where the electrically insulating member 40 is formed from compressed boric acid material, the M effect causing means 36 thus promotes or facilitates the melting of the fusible element 30 at a temperature which prevents damage to the electrically insulating member 40 due to long time overload conditions to which the fuse structure 10 may be subjected. When the fusible element 30 melts, as just described, and as the resulting arc between the melted ends of the fusible element burns back into the electrically insulating member or block 40, the insulating block evolves a gas, such as water vapor in the case of compressed boric acid, which aids in arc extinction and creates a relatively high pressure zone of increased dielectric strength which prevents a restrike of the arc once the arc is interrupted following a zero voltage point of the alternating current which is interrupted. The controlled gas evolution from the electrically insulating member 40 is facilitated during an interrupting operation by the intimate contact between the fusible element 30 and the material from which the electrically insulating member 40 is formed. The manner in which the electrically insulating member 40 is secured to the fusible element 30 enables the electrically insulating member 40 to confine the are which results during the operation of the fuse structure 10 and the intimate contact between the fusible element 30 and the electrically insulating member 40 serves to enhance the effect of the gases which are evolved during an arc interrupting operation and which assist in arc extinction. It is important to note that the pressing or forming of the electrically insulating member 40 directly on the associatedfusible element 30 in the manner previously described gives the electrically insulating member or block such relatively high mechanical and thermal strength that the block 40 does not burst or disintegrate during an interrupting operation of the fuse structure 10, but instead remains a solid expulsion pot device when subjected to the pressure and thermal conditions which result during an interrupting operation of the fuse structure 10, and also prevents the formation of a fulgurite along an axially extending portion of the fusible element 30. It is also important to note that the material from which the electrically insulating member 40 is formed is substantially non-tracking in the presence of any are which results during an interrupting operation of the fuse structure 10 to prevent the formation of an axially extending leakage path along the fusible element 30, which might otherwise permit restriking of the arc following a final interrupting operation of the fuse structure 10 after the alternating current which is interrupted passes through a zero voltage point on the alternating current waveform. Since the amount of gas evolution in the fuse structure 10 may be conveniently controlled by varying the axial length of the insulating block 40 and since the effect of the evolved gas is enhanced by the intimate contact between the fusible element 30 and the block 40 as previously mentioned, excessive gas pressure build-up is avoided during an interrupting operation of the fuse structure 10, which might otherwise cause the casing 20 to burst or be damaged.

In the operation of the fuse structure 10, when a relatively high or large abnormal or overload current starts to flow through the fusible element 30, the operation of the fuse structure 10 will differ from the low overload current operation previously described, in that a series or plurality of arcs will result when the fusible element 30 starts to melt, rather than only a single arc, such as results during the interruption of a relatively low overload current. More specifically, when a relatively high overload current starts to flow through the fusible element 30 of the fuse structure 10, the fusible element 30 will start to melt at a plurality of axially spaced locations which correspond to the restricted or reduced cross-sectional areas along the fusible element 30 as shown in FIG. 2, and a series of arcs and corresponding arc voltages will develop between the particles or drops of vaporized or melted fusible material from which the fusible element 30 is formed. As the melting of the fusible element 30 proceeds, the total arc voltage which develops along the fusible element 30 will increase quickly to a peak value to limit the value of overload current which flows to a value which is less than that which is available in the electrical circuit which is protected by the fuse structure 10. It is to be noted that during an interrupting operation of the fuse structure 10, when a relatively high overload current occurs, the arc-quenching filler material 72, which is dipsosed in the casing 20, will aid or assist in arc interruption by absorbing the thermal energy of the arc currents which develop during the operation of the fuse structure and form a fulgurite with vaporized material of the fusible element 30, as is well known in the fuse art. In addition, the presence of the filler material 72 assists in preventing excessive gas evolvement from the electrically insulating member 40 and excessive internal gas pressures within the casing 20 of the fuse structure l0.

Referring now to FIG. 3, there is illustrated a second embodiment of the invention in a fuse structure 100 which is similar to the fuse structure 10 previously described, except that the fuse structure 100 includes an additional electrically insulating member 40' which is disposed on and secured to the fusible element 30 between the single portion of the fusible element 30 and the other terminal member of the fuse structure 100 (not shown). The additional insulating member 40 is formed from the same type of material as the electrically insulating member 40 previously described, and is secured to the fusible element 30 in the same manner as the electrically insulating member 40 previously described.

The operation of the fuse structure 100 is similar to that of the fuse structure 10 previously described in detail, except that when the fuse structure 100 is called upon to interrupt relatively low overload currents, the fusible element 30 melts and burns back in both directions from the central portion thereof into both of the associated electrically insulating member 40 and 40' as previously explained in detail in connection with-the operation of the fuse structure 10, and provides the same operating advantages as the fuse structure 10 previously mentioned. It is to be noted that the presence of the additional electrically insulating member 40'in the fuse structure 100 provides additional controlled gas evolution which is particularly effective for relatively low overcurrent interrupting operations. It is to be noted that in a particular application, a plurality of electrically insulating members, such as the'insulating members 40 and 40' shown in FIG. 3, may be provided in a particular application and axially spaced from one another in order toprovide the desired amount of ,controlled gas evolution.

In the forming of the electrically insulating members 40 and 40 inthe fuse structures 10 or 100, respectively, it is to be understood that other types of suitable electrically insulating materials may be employed which evolve one or more gases in the presence of an arc, which-are adapted to withstand the relatively high temperatures or thermal conditions which may result during the operation of such fuse structures and which are also substantially non-tracking in the presence of an arc where desired in a particular application to provide various degrees of improvement in the fuse structures as disclosed. Such materials may include highly compressed calcium carbonate which evolves gases in the presence of an are which aid in arc extinction, such as carbon monoxide or carbon dioxide, which is adapted to withstand relatively high temperatures such as those which result during the operation of the disclosed fuse structures, and which is substantially non-tracking in the presence of an arc.

A high voltage current limiting fuse structure embodying the teachings of this invention has several advantages. .For example, a high voltage current limiting fuse structure,,including an electrically insulating member, as disclosed which is disposed on and secured to the associated fusible element, facilitates and provides improved low overcurrent interruption by confining the are which results during the operation of the fuse structureand providing a controlled amount of gas evolution which aids or assists in arc interruption and whose effect is enhanced by the intimate contact between the electrically insulating member and the associated fusible element. More specifically, the electrically insulating member in a fuse structure disclosed creates a high pressure zone or region of increased dieelectric strength during an interrupting operation which prevents restrike of an arc following the initial interruption of the are when the alternating current being carried by the fuse structure passes through a zero voltage point in its waveform. In addition, the electrically insulating member in a fuse structure as disclosed has a relatively high mechanical and thermal strength and remains solid and substantially intact to provide an expulsion pot device or means when subjected to the pressure and thermal conditions which may result during the operation of such a fuse structure. A further advantage of the applicants invention is that the electrically insulating member in a fuse structure as disclosed prevents the formation of a fulgurite along an axially extending portion of the associated fusible element, which may be particularly desirable during the interruption of relatively lower overcurrents. A final advantage of the applicants invention is that the relatively high density of the material from which an electrically insulating member in a fuse structure as disclosed also contributes to the relatively high mechanical and thermal strength of the electrically insulating member and to the effectiveness of the electrically insulating member during an arc interrupting operation when the desired gas evolution results.

I claim:

1. A high voltage current limiting fuse structure comprising a generally tubular, electrically insulating casing, terminal means disposed adjacent to each of the opposite ends of said casing, a fusible element disposed in said casing and extending axially between and connected between said terminal means, an elongated electrically insulating, normally solid member disposed on and secured to said fusible element, said electrically insulating member having one end disposed adjacent to the axially central portion of said fusible element and extending substantially around and axially along a portion of said fusible element toward one of said terminal means, said insulating member beingformed from a highly compressed material which is isostatically pressed on said fusible element in intimate contact with said fusible element and which is adapted to evolve a gas which aids in arc extinction in the presence of an arc which results when said fusible element melts, said last mentioned material being substantially nontracking in the presence of an arc, and a pulverulent arc quenching filler'disposed in said casing in contact with at least a portion-of said fusible element.

2. The combination as claimed in claim 1 wherein an additional elongated electrically insulating, normally solid member is provided disposed on and substantially around said fusible element with one end adjacent said axially central portion thereof and extending axially along said fusible element toward the other terminal means, said additional insulating member being formed from the same type of material as said first-mentioned insulating member.

3. The combination as claimed in claim 1 wherein an M effect causing means is disposed on said fusible element adjacent to said one end of said elongated electrically insulating member.

4. The combination as claimed in claim 2 wherein an M effect causing means is disposed on said fusible element between said first-mentioned elongated electrically insulating member and said additional elongated electrically insulating member.

5. The combination as claimed in claim 4 wherein the highly compressed material from which the electrically insulating member is formed in boric acid.

6. The combination as claimed in claim 4 wherein the highly compressed material from which the electrically insulating member is formed is calcium carbonate.

7. The combination as claimed in claim 6 wherein said boric acid which forms said insulating member is compressed on said fusible element by a minimum isostatic pressure of approximately 25,000 lbs. per square inch.

8. The combination as claimed in claim 1 wherein the highly compressed material from which the'electrically insulating member is formed is boric acid.

9. The combination as claimed in claim 4 wherein said boric acid which forms said insulating member is compressed on said fusible element by a minimum isostatic pressure of approximately 25,000 lbs per square inch.

10. The combination as claimed in claim 1 wherein the highly compressed material from which the electrically insulating member is formed is calcium carbonate. 

1. A high voltage current limiting fuse structure comprising a generally tubular, electrically insulating casing, terminal means disposed adjacent to each of the opposite ends of said casing, a fusible element disposed in said casing and extending axially between and connected between said terminal means, an elongated electrically insulating, normally solid member disposed on and secured to said fusible element, said electrically insulating member having one end disposed adjacent to the axially central portion of said fusible element and extending substantially around and axially along a portion of said fusible element toward one of said terminal means, said insulating member being formed from a highly compressed material which is isostatically pressed on said fusible element in intimate contact with said fusible element and which is adapted to evolve a gas which aids in arc extinction in the presence of an arc which results when said fusible element melts, said last mentioned material being substantially non-tracking in the presence of an arc, and a pulverulent arc quenching filler disposed in said casing in contact with at least a portion of said fusible element.
 2. The combination as claimed in claim 1 wherein an additional elongated electrically insulating, normally solid member is provided disposed on and substantially around said fusible element with one end adjacent said axially central portion thereof and extending axially along said fusible element toward the other terminal means, said additional insulating member being formed from the same type of material as said first-mentioned insulating member.
 3. The combination as claimed in claim 1 wherein an ''''M'''' effect causing means is disposed on said fusible element adjacent to said one end of said elongated electrically insulating member.
 4. The combination as claimed in claim 2 wherein an ''''M'''' effect causing means is disposed on said fusible element between said first-mentioned elongated electrically insulating member and said additional elongated electrically insulating member.
 5. The combination as claimed in claim 4 wherein the highly compressed material from which the electrically insulating member is formed in boric acid.
 6. The combination as claimed in claim 4 wherein the highly compressed material from which the electrically insulating member is formed is calcium carbonate.
 7. The combination as claimed in claim 6 wherein said boric acid which forms said insulating member is compressed on said fusible element by a minimum isostatic pressure of approximately 25,000 lbs. per square inch.
 8. The combination as claimed in claim 1 wherein the highly compressed material from which the electrically insulating member is formed is boric acid.
 9. The combination as claimed in claim 4 wherein said boric acid which forms said insulating member is compressed on said fusible element by a minimum isostatic pressure of approximately 25,000 lbs per square inch.
 10. The combination as claimed in claim 1 wherein the highly compressed material from which the electrically insulating member is formed is calcium carbonate. 